The present invention relates to a fluid actuated, double acting, linear actuating machine suitable for use in the punching of sheet metal, the cutting or bending of reinforcing steel rods or tubes, clinching and stamping, among other applications. More specifically, the invention pertains to such a machine of the .portable class integrally comprising a fluid pressure circuit with provisions for automatic retraction of the piston rod under fluid pressure.
The hydraulic linear actuating machine of the type under consideration comprises a piston slidably housed in a cylinder to define a pair of opposed fluid chambers therein, a piston rod coupled to the piston and slidably extending through one end of the cylinder, and a hydraulic circuit for the delivery and discharge of a hydraulic fluid under pressure into and out of the fluid chambers. The machine lends itself to use for punching, cutting and other applications by having an appropriate tool such as a punch or cutter replaceably mounted to the end of the piston rod. It is used in factories where steel sheets and steel rods are worked upon, as well as at sites of construction.
Conventionally, the linear actuating machine has been usually divided into two parts, one being the machine part comprising the cylinder, piston and so forth, and the other being a hydraulic unit comprising a hydraulic fluid reservoir and a pump. The machine part and the hydraulic unit have been intercommunicated via flexible conduits. For use at construction sites, the hydraulic unit has been placed in a convenient position, and the machine part has been carried about for working on desired parts.
The machine of such conventional make is not totally devoid of advantages. Being of relatively compact, lightweight construction, the machine part is convenient for use at factories or under other tidy circumstances. At construction sites, however, where many unexpected obstacles are to be encountered, much care has been needed in moving the machine part so that the flexible conduits may not be caught by such obstacles. Further the machine part has been movable only within the range permitted by the lengths of the flexible conduits, and the hydraulic unit itself has had to be replaced for moving the machine part beyond that range. An additional inconvenience is that the flexible conduits have had to be freed from their air contents at the cost of considerable time and labor preparatory to the commencement of actual operation.
The applicant is aware of a known portable hydraulic actuating machine in which the machine part and the hydraulic unit are integrally combined, as disclosed in U.S. Pat. No. 3,733,699. This known machine incorporates all of the cylinder, piston and pump into a single housing and relies on fluid pressure for the power stroke of the piston and on spring means for its return stroke.
Being free from elongate flexible conduits, the integrated actuating machine is highly portable and easy of use. Moreover, because of the shorter distance between pump and cylinder, the machine is good in piston response to pressurized fluid pressure. The portability and operability of the integrated actuating machine has earned it an increasing acceptance at construction sites in recent years.
The integrated actuating machine has one drawback, however. Relying on the force of a spring for its return stroke, the machine has had difficulties in retracting the piston rod in applications where much force is required for the return stroke. Take, for example, the use of the prior art machine as a punch. The machine will encounter great frictional resistance when the work being punched is very thick or when holes are created in the work with very small clearances for dimensional accuracy. The frictional resistance can be such that the piston rod has become unretractable solely by the force of the spring.
A conventional solution to this problem has been to mount a sleeve or sheathe of resilient material such as polyurethane rubber around the piston rod to which a punching tool is also attached. As the resilient sleeve becomes compressed upon punching, the energy thus stored in the sleeve is utilized for withdrawing the tool from within the hole.
This solution is unsatisfactory because of the relatively small force offered by the resilient sleeve. Additionally, the resilient sleeve itself represents an impediment to punching operation.
The applicant is not to claim that double acting machines have not so far existed in the art under consideration. Such a machine, complete with a manual directional control valve, has been used for the reshaping of reinforcing steel rods or the like that have been subjected to compressive or tensile bending. This double acting reshaping machine presupposes such use that the extension or retraction of the piston rod is manually controlled while visually observing the bend of a reinforcing steel rod or the like.
The above known double acting machine is not applicable to punching or like operations. When used as a punch, for example, the machine requires manual actuation of the direction control valve each time a hole is created. Further the electric motor driving the pump has had to be temporarily set out of operation for such manual actuation of the valve.